With the constant increase of mobile data services and emergence of new-type applications, the 3rd Generation Partnership Project (3GPP) organization has developed long-term evolution (LTE) specifications and LTE-Advanced (LTE-A) specifications. As the next generation cellular communication standard, an LTE or LTE-Advance system can operate in both Frequency Division Duplex (FDD) mode and Time Division Duplex (TDD) mode. In the FDD mode, the uplink and downlink employ a pair of frequency spectrums for data transmission; while in the TDD mode, the uplink and downlink channels share the same frequency, but occupy different time slots. Therefore, the TDD system has channel reciprocity, by which the downlink wireless channel information could be obtained with the knowledge got from the uplink channel.
Coordinated Multi-Point (CoMP) is one of the most important technologies for an LTE-A system to improve the cell edge User Equipment (UE)'s performance. In existing LTE-A specifications, there are defined four scenarios, wherein, for scenario 4, besides the macro eNB, the macro cell also includes several Remote Radio Headers (RRH) as transmission points, which share the same cell ID with the macro eNB.
Furthermore, the heterogeneous network (HetNet) with low power RRHs can further improve the cell throughput performance by decreasing the transmission distance between the transmitter and receiver. However, with shorter transmission range, the inter-cell interference among the RRHs is also increased, which decrease the performance, especially for those cell edge UEs. In such a case, CoMP is expected to be able to coordinate the data transmission among the RRHs together with the macro eNB, so that the cell edge UE's performance can be guaranteed.
FIG. 1 schematically illustrates a diagram of a system structure under scenario 4 in the prior art, which also shows the exemplary CoMP scenario. As illustrated in FIG. 1, in the macro cell shown in a solid ellipse, there are one macro eNB, four RRHs (i.e., RRH 0, RRH 1, RRH 2 and RRH 3) which are connected to the macro eNB by fibers, and four UEs (i.e., UE 0, UE 1, UE 2 and UE 3 ). In the macro cell, UE 0 is served by RRH 0; UE 1 can be served by RRH 1 and RRH 2 at the same time, wherein RRH 1 and RRH 2 constitute a cooperating set for UE 1; UE 2 can be served by RRH 3 and the macro eNB which constitute a cooperating set for UE 2; and UE 3 is served directly by the macro eNB. Therefore, in such a scenario, a UE can be served by a single transmission point or multiple transmission points in a cooperating set, but all of these transmission points share the same cell ID.
In 3GPP TS 36.213, there is defined a procedure of UE channel measurement and eNB scheduling, and for the purpose of illustration, the procedure is schematically shown in FIG. 2A. As illustrated, at step S201, UE measures the channel quality information (CQI) based on cell specific reference signal (CRS) that is sent from the eNB in a certain resource element (RE). For the TDD system and the transmission mode 7, 8 or 9, when PMI (Precoding Matrix Indicator)/RI (Ranking Indicator) reporting is disabled, the UE shall derive the channel measurement for computing CQI based on the CRS. The CRS is a pre-defined signal, pre-known to both the transmitter and the receiver, and thus the UE can derive the downlink channel CQI based on the received CRS. Then at step S202, UE reports the measured CQI as feedback information to the eNB through Physical Uplink Control Channel PUCCH (for periodic reporting) or through Physical Uplink Shared Channel PUSCH (for aperiodic reporting). After receiving the reported CQI, the eNB will schedule UEs in each TTI, based on the CQI, at step S203. The exemplary scheduling solutions can include, for example, Max C/I, round robin, proportional fairness and etc., which are well known in the art and thus will not be elaborated herein for the purpose of simplification. Then, the eNB will allocate resource blocks (RB) to the UE based on the scheduling results so as to send packets to the UE.
FIG. 2B schematically illustrates a diagram of mapping of downlink cell specific reference signals for two antenna ports and normal CP under 3GPP TS 36.211. In this figure, each block represents a resource element (RE), and in order to prevent the interference between the two ports, the resource elements which are used in antenna port 1 are not used for transmission in antenna port 0, and vice versa, as shown in FIG. 2B.
However, for the scenario 4 as mentioned hereinabove, wherein all transmission points share the same cell ID, such a solution is not appropriate. In such a scenario, all transmission points will send identical CRS, which causes that the UE can not differentiate these different transmission points in a macro cell at all. This means that the calculated CQI is overestimated and the reported CQI might even lead to a transmission failure, especially for those cell edge UEs.
Besides, in Chinese Patent Application No. 201110234923.X, entitled with “A Method and Apparatus for transmitting CQI”, and filed by Applicant CATR with the CSIPO on Aug. 16, 2011, there is disclosed a procedure of UE channel measurement and CQI-recalculation, which is schematically shown in FIG. 3A for the purpose of illustration. As illustrated in FIG. 3A, at step S301, UE receives channel state information reference signals (CSI-RS) from multiple points, wherein each transmission point will transmit a different CSI-RS to the UE. At step S302, the UE measures CQI for each of transmission points based on the received CSI-RS. Particularly, the measured CQIs include CQI for the main transmission point to UE and the CQIs for coordinated transmission points to UE. Specifically, the CQI for the main transmission point is the signal to noise plus interference (SINR) and the CQI for other coordinated transmission points are the relative CQIs to the main transmission point. The UE reports the measured CQIs to the eNB at step S303. Then, the eNB would re-calculate CQI based on reported CQI at step S304. Particularly, the eNB firstly modifies the UE's channel matrix with the help of the relative CQI measured in step S302; then, it calculates the noise and interference based on the CQI of the main transmission point; and lastly, it re-calculates the CQI by taking the number of co-scheduled UEs in the CoMP set into account.
FIG. 3B schematically illustrates a diagram of mapping of CSI-RS for eight antenna ports, CSI configuration 0 and normal CP under 3GPP TS 36.211, wherein CRS configuration is omitted for a reason of simplification. As illustrated, for each antenna port, only two REs are used to transmit CSI reference signal. Therefore, it is clear from FIG. 2B and FIG. 3B that CSI-RS is much sparser than CRS in time and frequency, which will lead to a coarse interference measurement.
On the other hand, in the disclosed solution, the CQIs are measured based on different CRI-RS from multiple transmission points and there will be a large number of CQIs to be reported. Therefore, the uplink overhead is significantly increased. In addition, the CSI-RS itself introduces extra downlink overhead.
Moreover, it is the most important that, under the current standard (3GPP TS 36.213), it requires the UE to calculate CQI based on CRS when PMFRI report is disabled but this disclosed solution does not support it at all. In addition, the introduction of the CSI-RS means the modification to the UE and thus it is incompatible with existing available UEs, for example those UEs measuring CQI based on CRS.
Therefore, there is urgently needed a new CQI estimation scheme in the art.